Boiler Instrumentation and Controls

Abstract

Instrumentation and controls in a boiler plant encompass an enormous range of equipment from simple industrial plant to the complex in the large utility station.

The boiler control system is the means by which the balance of energy & mass into and out of the boiler are achieved. Inputs are fuel, combustion air, atomizing air or steam &feed water.

Of these, fuel is the major energy input. Combustion air is the major mass input, outputs are steam, flue gas, blowdown, radiation & soot blowing.

CONTROL LOOPS

Boiler control systems contain several variable with interaction occurring among the control loops for fuel, combustion air, & feedwater . The overall system generally can be treated as a series of basic control loops connected together. for safety purposes, fuel addition should be limited by the amount of combustion air and it may need minimum limiting for flame stability.

Combustion controls

Amounts of fuel and air must be carefully regulated to keep excess air within close tolerances-especially over the loads. This is critical to efficient boiler operation no matter what the unit size, type of fuel fired or control system used.

Feedwater control

Industrial boilers are subject to wide load variations and require quick responding control to maintain constant drum level. Multiple element feed water control can help faster and more accurate control response.

DIFFERENT TYPES OF COMBUSTION CONTROLS

There are three general types of combustion control schemes used today: They are series, parallel & series-parallel controls.

In series control, variations in steam header pressure(the master control signal) cause a change in combustion air flow which in turn results in a sequential change in fuel flow. This type of control is limited to small boilers having relatively constant steam load & burning fuel.

In parallel control, variation in steam pressure simultaneously adjusts both fuel & air flows. This method is common to any size boilers.

In series-parallel, variation in steam pressure set points are used to adjust the fuel. Flow to the above boiler since steam flow is directly related to heat release of the fuel and hence the air flow, the steam flow can be used as an index of the required combustion air.

HARDWARES USED IN COMBUSTION CONTROL

The control hardware used to carryout the above schemes include ON/OFF controls, positioning & metering systems.

(a) ON/OFF controls:

Are still used in many industries but are generally used in small water tube boilers. When the pressure drops to a present value, fuel & air are automatically fed into the boiler at predetermined rate until pressure has risen to its upper limit.

(b) Positioning systems:

Respond to changes in header pressure by simultaneously positioning the forced draft damper and fuel valve to a predetermined alignment. This is not used in liquid , gaseous fuel – fired boilers.

(c) Metering control system:

In this system control is regulated in accordance with the measured fuel and air flows. This maintains combustion efficiency over a wide load ranges & over long period of time.

Both metering & positioning control systems use steam header pressure as their primary measured variable & as a basis for firing rate demand. A master pressure controller responds to changes on header pressure & positions the dampers to control air flow and fuel valve to regulate fuel supply.

FEEDWATER CONTROL

Feedwater control is the regulation of water to the boiler drum. It provide a mass accounting system for steam leading and feedwater entering the boiler. The water is admitted to the steam drum and after absorbing the heat from furnace generates the steam produced by the boiler.

Proper boiler operation requires that the level of water in the steam drum should be maintained within certain band. A decrease in this level may uncover boiler tubes, allowing them to become overheated. An increase in the level of water may interfere with the internal operation of internal devices in the boiler drum. It is important to made that the water level in the boiler drum must be above 50% all the time.

The water level in the boiler drum is related to, but is not a direct indicator of , the quantity of water in the drum. At each boiler load, there is different volume in the water that is occupied by steam bubbles. So if load is increased there are more steam bubbles and this cause water to ‘swell’ or rise, rather than fall because of added water usage.